Allergic bronchopulmonary aspergillosis, known to be associated with cystic fibrosis in older patients, occurred in 7 young atopic children with cystic fibrosis. The diagnosis was suggested by the onset of, or the increase in, asthmatic symptoms accompanied by major chest x-ray changes ranging from total collapse of a lung or lobe to extensive but changing areas of consolidation. Each of the children had a blood eosinophilia, positive type I skin tests to Aspergillus fumigatus, and reversible airways obstruction. Most had a positive type III skin test and circulating precipitins to A. fumigatus, with raised IgE levels which contained specific antibodies to the fungus on radioallergosorbent (RAST) test. None had advanced suppurative chest disease of cystic fibrosis. None was given specific antifungal agents; two received systemic treatment with corticosteroids, the other received additional drugs for their asthma. Two developed total collapse of one lung, one child being only 2 years old. Five have had recurrences of pulmonary shadowing typical of allergic aspergillosis but are not showing significant progression of their cystic fibrosis lung disease. Our experience suggests that there should be an increased awareness of this condition, particularly its association with extensive pulmonary collapse or consolidation in children with cystic fibrosis who are atopic.